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OTL amps for guitar duty
OTL amps for guitar duty
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Old 7th February 2017, 05:55 AM   #11
AquaTarkus is offline AquaTarkus  Canada
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Yes, most guitar amps are atrocious from a hifi design perspective. The ones with more sag tend to be used more often for jazz and blues, rather than heavy rock and metal. Then you have Hiwatts whose OT had a 3.6Hz to 112kHz response! Anyway let's get back on topic :-)

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Old 7th February 2017, 04:40 PM   #12
atmasphere is offline atmasphere  United States
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The funny thing about guitar players is how picky they are about their 'sound'. More so than audiophiles. But 80% of them play either a Marshall (or clones of it) or a Fender (or clones of it), but sculpting their 'sound' using effects pedals. The remaining 80% use something else- smaller combo amps, Ampeg, Vox or the occasional Sunn.

These days, many guitar players are really relying on their pedals. When you meet a player like that, an OTL guitar amp works fine for them. OTL guitar amps can have a soft clipping character that is very nice. But I have found that the preamp design is far more important!
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Old 8th February 2017, 05:15 AM   #13
AquaTarkus is offline AquaTarkus  Canada
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Quote:
Originally Posted by atmasphere View Post
The funny thing about guitar players is how picky they are about their 'sound'.
Yes, because that sound is their "voice". However, IMO the starting point should be a really good amp, and not just a mass-produced Fender, Marshall, Vox knock off. Key points for the tone are first the transformers , then the tubes, followed by the caps and resistors. Cheeping out on those will provide a less than stellar experience. Once you have a real good quality amp, then you have the basis for a great sound and any pedals may just be used to add an optional dash of extra seasoning.



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Old 8th February 2017, 10:01 AM   #14
SemperFi is offline SemperFi  Wake Island
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Old 8th February 2017, 04:48 PM   #15
atmasphere is offline atmasphere  United States
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Quote:
Originally Posted by AquaTarkus View Post
Yes, because that sound is their "voice". However, IMO the starting point should be a really good amp, and not just a mass-produced Fender, Marshall, Vox knock off. Key points for the tone are first the transformers , then the tubes, followed by the caps and resistors. Cheeping out on those will provide a less than stellar experience. Once you have a real good quality amp, then you have the basis for a great sound and any pedals may just be used to add an optional dash of extra seasoning.



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I've run into those that maintained that cheap carbon composition resistors are the trick parts for a guitar amp.

At any rate, if you want to add ferro-magnetics as a coloration to the sound of an OTL, its easy to do without using an output transformer! All you have to do is get a cheap interstage transformer and you're all set. That will allow you a very simple circuit.

The Edcor 3X15K-0 is a good example (having already used it successfully). I used this part to drive a Circlotron output section that was lightly biased (so no need for a bias control or DC Offset control), driven by a single-ended 6SN7 on the primary side of the transformer.

The amp I built can be seen on the Atma-Sphere facebook page

https://www.facebook.com/AtmaSphereM...type=3&theater

The photo is of the complete amp. The controls and input are on the other side of the chassis. Its all handwired, point to point. In order to be useful though with only four 6AS7Gs, it needs a speaker load of 16-32 ohms. There are several ways to do that!

My first attempt was a combo cabinet (which employed a separate power supply mounted on the bottom of the cabinet, power amp chassis mounted near the bottom and close to the drivers with the preamp chassis on top) that used four 8-ohm 8" Jensens in series for 32 ohms, arranged as a line-source array. This allowed for consistent sustain whether you were close or further away, and also allowed others in the band to more easily hear the amp without having to blast it while close up to the amp. You could also build it in a dual 10 or 12 configuration like a Fender Twin- that's my next attempt.

What I found with working with these amps is the preamp is far more important than the amp! The first preamps I built had far too much gain (6 volume controls, two channels plus a Fender style tone stack). You could really get some crunch out of it, but most players had no idea what to do with all the volume controls even though I had them pretty clearly marked.

The second preamp had less gain and less volume controls but still went too far. So I did the single-chassis version you see at the link, which dumped the midrange control (mostly out of space concerns- I was using a chassis that was designed to be a power amp and power supply). The single-ended driver configuration really helped with reducing the space needed by the driver circuit.

If you put the amp on an 8 ohm load it tends to make too much bass because the power amp does not employ any negative feedback. It would cut through a bit more if that were added, but I think that would work best if done as a presence control like you see on Marshalls.

Obviously its a work in progress! The amp section worked great from the very beginning; putting a proper preamp to it has been the issue.
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Old 9th February 2017, 02:49 AM   #16
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by AquaTarkus View Post
Please excuse any typpos.
There's only one "p" in "typos". Oh, the irony!

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Old 9th February 2017, 02:56 AM   #17
AquaTarkus is offline AquaTarkus  Canada
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Deliberate irony ;-)

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Old 9th February 2017, 03:21 AM   #18
multi-volti is offline multi-volti  United States
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Typpopotamuses?
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Old 9th February 2017, 03:22 AM   #19
multi-volti is offline multi-volti  United States
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Quote:
Originally Posted by atmasphere View Post
I've run into those that maintained that cheap carbon composition resistors are the trick parts for a guitar amp.

At any rate, if you want to add ferro-magnetics as a coloration to the sound of an OTL, its easy to do without using an output transformer! All you have to do is get a cheap interstage transformer and you're all set. That will allow you a very simple circuit.

The Edcor 3X15K-0 is a good example (having already used it successfully). I used this part to drive a Circlotron output section that was lightly biased (so no need for a bias control or DC Offset control), driven by a single-ended 6SN7 on the primary side of the transformer.

The amp I built can be seen on the Atma-Sphere facebook page

https://www.facebook.com/AtmaSphereM...type=3&theater

The photo is of the complete amp. The controls and input are on the other side of the chassis. Its all handwired, point to point. In order to be useful though with only four 6AS7Gs, it needs a speaker load of 16-32 ohms. There are several ways to do that!

My first attempt was a combo cabinet (which employed a separate power supply mounted on the bottom of the cabinet, power amp chassis mounted near the bottom and close to the drivers with the preamp chassis on top) that used four 8-ohm 8" Jensens in series for 32 ohms, arranged as a line-source array. This allowed for consistent sustain whether you were close or further away, and also allowed others in the band to more easily hear the amp without having to blast it while close up to the amp. You could also build it in a dual 10 or 12 configuration like a Fender Twin- that's my next attempt.

What I found with working with these amps is the preamp is far more important than the amp! The first preamps I built had far too much gain (6 volume controls, two channels plus a Fender style tone stack). You could really get some crunch out of it, but most players had no idea what to do with all the volume controls even though I had them pretty clearly marked.

The second preamp had less gain and less volume controls but still went too far. So I did the single-chassis version you see at the link, which dumped the midrange control (mostly out of space concerns- I was using a chassis that was designed to be a power amp and power supply). The single-ended driver configuration really helped with reducing the space needed by the driver circuit.

If you put the amp on an 8 ohm load it tends to make too much bass because the power amp does not employ any negative feedback. It would cut through a bit more if that were added, but I think that would work best if done as a presence control like you see on Marshalls.

Obviously its a work in progress! The amp section worked great from the very beginning; putting a proper preamp to it has been the issue.
Cool...thanks for the description.
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Old 9th February 2017, 05:32 AM   #20
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by AquaTarkus View Post
Most guitar amp designers and builders will argue that the OT is an essential part of the tone.
I have heard this many times, and I do believe it's quite plausible. The logic being, "tone" actually is a misnomer for "timbre" in this context, and anything that creates roughly 10% or more harmonic distortion has the potential to change the timbre.

There are only a few components that can potentially create that much THD in a guitar amp, and those include the actual valves, the speaker itself, and the output transformer.

So it's certainly plausible that the output transformer contributes to guitar amp "tone". But it bothers me that I've never heard of anyone actually doing an experiment to prove it.

Oh, we have plenty of people saying that their guitar amp sounded better after they put in a $200 Mercury Magnetics transformer or something like that. Trouble is, there is no AB comparison possible, and human psychology dictates that you usually will think something is better if you just spent $200 on it. So these sorts of claims more or less fall into the "that could possibly be true, but it could also just be the placebo effect" category.

In principle, it doesn't seem too hard to come up with an experiment to test if guitar amp output transformers actually contribute to the amps timbre. Say, drive a guitar speaker with increasing amounts of sine-wave 500 Hz power, first from a low-distortion solid-state amp direct-coupled to the speaker, and then from the same amp via a 1:1 ratio output transformer made with the same-size iron as the actual valve amp OT. Or even two back-to-back OTs, one stepping the voltage up, the other stepping it back down, for an overall 1:1 ratio (less losses).

If you hear significantly different timbre via the transformers, the hypothesis is proved. If you don't, well, it turns out to be yet another amp-guru myth. Either way, we all learn something useful!

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